1. Field of the Invention
The present invention relates to shoe seals for sealing the space between a floating roof and an inner tank wall within a storage tank, and to methods of installing such shoe seals within the storage tank.
2. History of the Prior Art
It is standard practice in the field of large storage tanks such as oil storage tanks having floating roofs to provide the floating roof with a seal. Such seals act to seal the space between the outer rim of the floating roof and the inner tank wall, while at the same time allowing the floating roof to rise or drop as the height of the liquid product within the storage tank varies. Such seals are necessary in order to prevent harmful hydrocarbon vapors from escaping through the space between the floating roof and the inner tank wall and entering the atmosphere.
Seals for floating roofs within storage tanks can assume a variety of different configurations. One such arrangement is shown in U.S. Pat. No. 4,308,968 of Thiltgen et al., which patent issued Jan. 5, 1982 and is commonly assigned with the present invention. The arrangement shown in the Thiltgen et al. patent includes two different seals, the first being a primary seal and the second being a backup or secondary seal. The use of a secondary seal such as the one shown in the Thiltgen et al. patent is sometimes necessary, due to the nature of the primary seal and to environmental requirements. The sealing arrangements shown in the Thiltgen et al. patent utilize vapor barriers in combination with flexible metal plates and wiper blades. The vapor barriers, which are common in many floating roof seals, comprise one or more layers of fabric which are generally impermeable to vapors from the liquid product stored in the tank.
One type of floating roof seal which has been found to be quite effective, and which usually eliminates the need for a secondary seal, is the shoe seal. Shoe seals employ a shoe in the form of a series of joined-together plates which are disposed against the inner wall of the tank and which are supported by the outer rim of the floating roof. A vapor barrier extending between the outer rim and the shoe provides an effective barrier to vapors from the liquid product in the tank, inasmuch as the lower portion of the shoe extends into the product.
Examples of shoe seals are provided by U.S. Pat. No. 2,981,438 of Heisterberg, which patent issued Apr. 25, 1961, U.S. Pat. No. 3,167,206 of Nelson, which patent issued Jan. 26, 1965, and U.S. Pat. No. 4,130,217 of Hills et al., which patent issued Dec. 19, 1978. In Heisterberg et al. the sealing mechanism is provided with a combination weatherhood and wax trough. The shoe is forced against the inner tank wall by spring-loaded pistons mounted within the outer rim of the floating roof. In Nelson, the shoe is suspended from the outer rim of the floating roof by a pivoting hanger structure designed to force the shoe against the inner tank wall. In Hills et al. various different members including springs are employed to maintain the shoe against the inner tank wall.
In spite of the advantages of shoe seals, presently known seals of this type typically suffer from certain limitations which may limit their effectiveness, their versatility and their ease of installation. For best performance, the shoe should be hung from the outer rim of the floating roof using hanger assemblies which support the shoe from the roof while at the same time allowing the shoe ample freedom to move toward or away from the roof as necessary to span the gap between the floating roof and the inner tank wall. Such hanger assemblies are capable of accommodating variations in the space around the outer rim of the roof which can result from construction practices and from settling of the tank foundation. Present shoe seal designs also typically lack apparatus for maintaining the shoe in contact with the inner tank wall in an effective manner, particularly when combined with the advantages of pivoting hanger assemblies. Such conventional designs typically provide some urging of the shoe toward the inner tank wall without providing the uniform distribution of force on the shoe which may be necessary to maintain the shoe in engagement with the inner tank wall along substantially the entire length of the shoe. Various spring-like devices have proven unsuccessful, either because such devices become easily misaligned or dislocated with roof movement, or because such devices do not urge the shoe with the uniformity of pressure necessary to accommodate tank wall variations.
Further problems arise in connection with presently known shoe seals, from the standpoint of installation. Those seals requiring welding of various parts thereof to the floating roof, for example, necessitate that the tank be completely drained and the vapors removed therefrom before the seal can be installed on the roof. It would be highly advantageous to provide a shoe seal which can be installed while the tank is in service with the liquid product present therein. This dictates that the seal be installed relatively easily, and in any event without the need for welding or other procedures which would pose a danger of igniting a hydrocarbon product stored therein. For that matter, it would be desirable to be able to assemble those portions of the shoe seal which are preassembled outside of the tank without the need for welding.